Dual suppression of Glossina pallidipes using entomopathogenic fungal-based biopesticides and sterile insect technique

Abstract

Tsetse flies and trypanosomosis significantly impact bovine production and human health in sub-Saharan Africa, exacerbating underdevelopment, malnutrition, and poverty. Despite various control strategies, long-term success has been limited. This study evaluates the combined use of entomopathogenic fungi (EPF) and the sterile insect technique (SIT) to combat tsetse flies. Eleven EPF isolates were tested against teneral males of Glossina pallidipes, focusing on mortality rates, radial growth, and impacts on fly fitness. Temperature effects on conidial growth, sporulation, and spore yield of SIT-compatible/tolerant strains were also assessed. The fungal isolates significantly influenced mortality rates in both unirradiated and irradiated (SIT-treated) males (p < 0.0001). Metarhizium anisopliae strains ICIPE 20, ICIPE 32, ICIPE 41, ICIPE 62, ICIPE 78, and Beauveria bassiana ICIPE 603 showed higher SIT compatibility/tolerance with LT₅₀ values of 11-30 days, compared to other more virulent isolates with LT₅₀ values of 4-9 days. Temperature significantly affected the radial growth of SIT-compatible EPF strains (p < 0.0001), with M. anisopliae ICIPE 78 exhibiting the fastest conidia growth at 25°C. Spore yield varied significantly across temperatures (15-40°C), and the thermal range for conidia germination of SIT-compatible strains was 8.1-45.4°C, with an optimal range of 26.7-31.1°C. Moreover, infected unirradiated females and irradiated males (donors) successfully transmitted conidia to untreated flies (receivers) without significant differences in survival rates (p = 0.6438) and no observed sex dimorphism. Our findings highlight the potential of combining EPF and SIT as a novel dual approach that could effectively and synergistically suppress tsetse fly populations.

Keywords: African animal trypanosomiasis (AAT); Beauveria bassiana; Glossina pallidipes; Metarhizium anisopliae; area-wide integrated pest management (AW-IPM); human African trypanosomiasis (HAT).

Figure 1

Pathogenicity and virulence of the different entomopathogenic fungal (EPF) isolates against tenerals of Glossina pallidipes (n = 30). (A) Cumulative mortality of EPF-challenged unirradiated G. pallidipes males. (B) Cumulative mortality of EPF-challenged irradiated G. pallidipes males. (C) Median lethal time (LT₅₀) of EPF-challenged unirradiated G. pallidipes males. (D) The median lethal time (LT₅₀) of EPF-challenged irradiated G. pallidipes males. Means were separated using Tukey’s HSD post hoc test. Error bars indicate the standard error of the means, and the bars/means followed by different letters are significantly different (p > 0.05).

Figure 2

Comparison of the relative growth rates of the SIT-compatible Metarhizium anisopliae strains (ICIPE 20, ICIPE 41, ICIPE 62 and ICIPE 78) and Beauveria bassiana ICIPE 603, at temperatures ranging from 15 to 40°C (n = 120).

Figure 3

Relationship between radial growth rates of the SIT-compatible EPF and temperature. (A) Relative radial growth rates of M. anisopliae ICIPE 20, 41, 62, 78 and B. bassiana 603 using the Cardinal Temperature Model with Inflection (CTMI) between 15 and 40°C. (B) Relative radial growth rates of M. anisopliae ICIPE 20, 41, 62, 78 and B. bassiana 603 using the Lactin 1 between 15–40°C. The CTMI models are represented by the blue continuous line, Lactin 1 models are represented by the red continuous line while the linear model is represented by the black continuous line (n = 120; Tukey’s HSD test).

Figure 4

Horizontal transmission of SIT-compatible Metarhizium anisopliae conidia in Glossina pallidipes (analysis by ANOVA followed by Tukey’s HSD post hoc test; p > 0.05, *** p < 0.001, n = 10, replicated thrice). Error bars indicate the standard error of the mean and the bars with different lowercase letters are significantly different from each other. (A) Variation of M. anisopliae (ICIPE 20 and ICIPE 78) conidia transmission from irradiated and unirradiated G. pallidipes males (conidia donors) to unirradiated virgin G. pallidipes females (conidia receivers) across time. (B) Variation of M. anisopliae (ICIPE 20 and ICIPE 78) conidia transmission from EPF-exposed G. pallidipes females (conidia donors) to irradiated and unirradiated G. pallidipes males (conidia receivers) across time.

Figure 5

Kaplan–Meier curve illustrating survivorship over time. (A) Survival curves of the conidia donors (virgin G. pallidipes females) over time. (B) Survival curves of the conidia donors (irradiated G. pallidipes males) over time. (C) Survival curves of the conidia receivers (irradiated G. pallidipes males) over time. (D) Survival curves of the conidia receivers (virgin G. pallidipes females) over time after exposure to irradiated-EPF-exposed G. pallidipes males (Mantel-Cox log-rank χ² test, p < 0.05, n = 30).

Figure 6

Mean lethal time (LT₅₀) between Glossina pallidipes conidia donors and conidia receivers (both male and female) (p > 0.05, n = 10). Error bars indicate the standard error of the mean and the bars with different lowercase letters are significantly different from each other.